A tricobalt device is a high-yield explosive used rarely by the United Federation of Planets and that were monitored under the auspices of the Second Khitomer Accords.
In 2155 in the mirror universe, the Tholians use tricobalt devices to create an interphasic rift through which they sent a distress call in order to draw ships to them. They succeeded in drawing the primary universe's USS Defiant through the rift, allowing the Tholians to capture Defiant. (ENT episode: "In a Mirror, Darkly")
In 2268, an Arne Darvin from 2373 travelled back in time with a tricobalt device contained in a tribble with the hope that it would detonate near James T. Kirk. His efforts would be stopped by the crew of Deep Space 9. (DS9 episode: "Trials and Tribble-ations")
In 2371, USS Voyager was deployed with tricobalt weapons which would be used to destroy the Caretaker after Voyager was pulled into the Delta Quadrant. (VOY episode: "Caretaker")
In 2373, the Maquis were able to obtain tricobalt weapons, which some feared would be used in a sneak attack on Cardassia Prime. (DS9 episode: "Blaze of Glory")
In 2381, the USS Titan was carrying 8 tricobalt devices which they used to force the Null back into subspace. (TTN novel: Synthesis)